Nitride-base compound semiconductors are known to show direct transition over the entire compositional region thereof (for example, AlN, GaN, InN and mixed crystals thereof), have wide band gaps, and are known as materials for blue or violet light emitting diode, in other words, materials for short-wavelength light emitting elements.
The nitride-base compound semiconductors, however, have crystal structures of the hexagonal system, for which there is no lattice-matched substrate crystal unlike the conventional Group III-V compound semiconductors. Therefore, they are generally grown on the surface of sapphire substrate (SiC or Si is also adoptable) having the hexagonal structure.
However, since crystal lattice of the (0001) surface of the sapphire substrate and that of the (0001) surface of the nitride-base compound semiconductor differ in the lattice constants, there is lattice mismatch between them. Thus, growth of the nitride-base compound semiconductor layer onto the sapphire substrate results in an insufficient crystallinity as a continuous thin film, and fails in obtaining an epitaxial film having a low threading dislocation density.
Presence of a high density of such threading dislocation results in degradation in light emitting characteristics, when the nitride-base compound semiconductor is used as a material for high-luminance light emitting diodes and semiconductor lasers.
For this reason, a GaN thin film having a predetermined thickness is grown on the sapphire substrate, and thereafter a stripe-patterned or a mesh-patterned mask composed of a thin film of SiO2, SiN or metal is formed. Thereafter, re-growth of GaN under specific conditions can allow GaN to grow only on the exposed GaN portions, without causing crystal growth on the mask.
In this process, on the mask, GaN grows laterally and fuses on the top surface of the mask, so that the entire surface can be covered with GaN, and finally a flat continuous thin film of GaN can be produced (see Non-Patent Documents 1 and 2).
The threading dislocation density can be largely reduced in the GaN continuous thin film epitaxially grown laterally on the mask, when compared to in GaN continuous thin films formed by the general manufacturing methods.
As a technique similar to as described in the above, there has been also proposed a technique of reducing the threading dislocation, by forming steps on the sapphire substrate or the GaN film so as to allow the lateral growth.    [Non-Patent Document 1] A. Usui, H. Sunakawa, A. Sakai and A. Yamaguchi, “Thick GaN epitaxial growth with low dislocation density by hydride vapor phase epitaxy”, Jpn. J. Appl. Phys., 36 (7B) 1997.    [Non-Patent Document 2] A. Sakai, H. Sunakawa and A. Usui, “Defect structure in selectively grown GaN films with low threading dislocation density”, Appl. Phys. Lett., 71 (16) 1997